The present invention relates to a magnetic tape cassette, particularly to a magnetic tape cassette in which friction sheets are provided between the body of the cassette and hubs on which a magnetic tape is wound.
A magnetic tape cassette is widely used as a record/playback medium with an audio apparatus such as a magnetic tape recorder. In the cassette, a pair of flangeless hubs on which a magnetic tape is wound are rotatably supported in a cassette body composed of an upper and a lower half portion. When the magnetic tape cassette is used with a tape recorder, the magnetic tape is typically repeatedly and frequently run and stopped as the recorder is put in a recording or playback mode, a stop mode, a rapid running mode, a stop mode, etc. For that reason, friction sheets are provided on the inside surfaces of the upper and lower body portions of the cassette. The friction sheets have surfaces having properties so as to maintain good running properties of the magnetic tape and to apply an appropriate tensile force to the tape during the running thereof and prevent the tape from undergoing slackening or the like when the tape is stopped.
For this purpose, each of the friction sheets is a thin sheet of a soft synthetic resin. Some such friction sheets are processed for prevention of static electric charging, and are embossed, curved appropriately, or are provided with protruding portions of appropriate height. For a friction sheet of very small thickness, such protruding portions serve to effectively increase the thickness of the sheet and enhance its function of supporting the hubs and the magnetic tape to prevent them from moving in the direction of thickness of the cassette. However, friction sheets having such protruding portions have certain problems, as will now be described.
Friction sheets having protruding portions can be broadly divided into two types, as shown in FIGS. 6 and 7. The friction sheet 30 of the type shown in FIG. 6 has two protruding portions 37 and 38 having nearly rectangular or U-shaped cross sections and straightly extending in parallel with each other in the longitudinal direction of the sheet so that the protruding portions extend to both ends 3a and 3b of the sheet and the hub holes 4 of the sheet are located between the protruding portions
The friction sheet 40 of the type shown in FIG. 7 has two protruding portions 47 and 48 having nearly rectangular or U-shaped cross sections and straightly extending in parallel with each other in the longitudinal direction of the sheet so that the protruding portions do not reach the ends 3a and 3b of the sheet and the hub holes 4 of the sheet are located between the protruding portions, namely, the sheet is flat at both ends thereof, as differs from the former type shown in FIG. 6.
Since the protruding portions 37 and 38 of the friction sheet 30 of the type shown in FIG. 6 extend throughout the whole length of the sheet, the sheet has an advantage that the protruding portions are capable of functioning well with a magnetic tape wound to a large diameter. However, the sheet has a disadvantage that since the protruding portions are open at both ends 3a and 3b of the sheet, the protruding portions tend during prolonged use to be elongated in the direction B of the width of the portions at and near the ends 3a and 3b and to be reduced in height from a preset value so that the supporting function of the protruding portions is reduced.
Since the protruding portions 47 and 48 of the friction sheet 40 of the type shown in FIG. 7 are closed at the ends of the portions inside the ends of the sheet, the sheet does not have the disadvantage that the protruding portions are likely to be elongated in the direction B of the width of the portions. However, the sheet has a problem in that wrinkles 50 tend to form at and near the ends of the protruding portions. The smaller the distance between the end of each of the protruding portions 47 and 48 and the end 3a or 3b of the sheet is, the more likely the wrinkles 50 are to occur. Due to the wrinkles 50, the sheet 40 undergoes strain, causing the sheet to be deformed to warp at or near the ends 3a and 3b thereof, thereby decreasing the degree of flatness of the sheet and deteriorating the function of the sheet in such a manner that the deformed parts of the sheet come into contact with the magnetic tape.